(1) Field of the invention
The present invention generally relates to an optical image writing unit, and more particularly to an optical image writing unit applicable to a xerographic image forming apparatus.
(2) Description of related art
An image writing unit provided in a xerographic image forming apparatus for forming multicolor images is provided with one or a plurality of light beams used for optically writing images. The light beams are modulated in accordance with image data, and scan the surface of a photoconductive body (i.e. a photoconductive drum, or a photoconductive belt) moved at a predetermined speed, so that electrostatic latent images are formed on the photoconductive body. The electrostatic latent images are developed so as to be visualized in colors, and a multicolor image is obtained by superposing developed color images. In this type of image writing unit, an optical writing position on the photoconductive body, to which position each light beam is projected, varies from an original optical writing position to which the light beam should be projected, due to a variation of the moving speed of the photoconductive body. As a result of this, colors are offset in the multicolor image, and thus the quality of the multicolor image deteriorates.
In a case where a ladder chart having black and red lines alternately arranged at a constant pitch is recorded under a condition in which there is no variation of the moving speed of the photoconductive body, the black lines BK and the red lines RD are arranged at the constant pitch in a recorded image, as shown in FIG. 1. On the other hand, in a case where there is a variation of the moving speed of the photoconductive body, a distance between black and white lines GK and RD adjacent to each other varies in the recorded image corresponding to the ladder chart, as shown in FIG. 2.
The variation of the moving speed of the photoconductive body is mainly caused by eccentricities of the photoconductive drum, driving rollers for driving the photoconductive belt, and pulleys and gears provided in a driving mechanism of the photoconductive body. In a case where there are eccentricities of the pulleys, the gears and the like, the moving speed of the photosensitive body periodically varies. When the moving speed of the photoconductive body periodically varies, an amount of displacement between the actual optical writing position and the original optical writing position periodically varies also. This displacement is referred to as a writing position displacement .DELTA.x.
The following optical image writing units have been conventionally proposed to eliminate the above problem caused by the variation of the moving speed of the photoconductive drum.
A first conventional optical image writing unit is disclosed in Japanese Patent Laid Open Publication No.1-257980. The first conventional optical image writing unit has two light beams used for optically writing images. In the first conventional optical image writing unit, a variation cycle of moving speed (position) of the photosensitive body is measured, and a distance between optical writing positions, on the photoconductive drum, to which positions two light beams are respectively projected is determined based on the measured variation cycle of the moving speed of the photosensitive drum.
A second conventional optical image writing unit is disclosed in Japanese Laid Open Publication No.2-156260. In the second conventional optical image writing unit, image forming units are arranged around a transfer drum at positions corresponding to the same phase in a variation cycle of the moving speed of the transfer drum.
However, regarding the first conventional optical image writing unit, to determine the distance between optical writing positions of two light beams on the photosensitive drum, an experimental unit having the same layout as an actual unit must be made. In the experimental unit, the variation cycle of the moving speed has to be measured. Thus, a large amount of time is required to design the optical image writing unit. In addition, the distance between two optical writing positions on the photosensitive drum also depends on a layout of other units, such as developing units. In a case where the distance between two optical writing positions is determined based on only the variation cycle of the moving speed of the photoconductive body, the developing units are not always efficiently arranged around the photoconductive body. Thus, there is a case where the size of the optical image writing unit increases.
In the second conventional optical image writing unit, the disadvantages caused by the variation of the moving speed of the transfer drum are eliminated. In this case, if there is no variation of a moving speed of a photoconductive body in each of the image forming units arranged around the transfer drum, color images formed by the image forming units can be accurately superposed on each other on the transfer drum. However, there is variation of the moving speed of the photoconductive body in each image forming unit. Thus, colors in the multicolor image are offset by the variation of the moving speed of the photoconductive body in each image forming unit.